7.9.7 Operation principleThe sampled analog phase currents are pre-processed and for each phase current the RMSvalue is derived. These phase current values are fed to the thermal overload protection, onetime constant LFPTTR/LCPTTR (26) function. The temperature is displayed either inCelsius or Fahrenheit, depending on whether LFPTTR/LCPTTR (26) function is selected.From the largest of the three-phase currents a final temperature is calculated according tothe expression:2final refrefI TIæ öQ = ×ç ÷ç ÷è øEQUATION1167 V1 EN (Equation 56)where:I is the largest phase current,Iref is a given reference current andTref is steady state temperature rise corresponding to IrefIf this temperature is larger than the set operate temperature level, TripTemp, a PICKUPoutput signal is activated.The actual temperature at the actual execution cycle is calculated as:( )1 1 1tn n final n etD-- -æ öQ = Q + Q - Q × -ç ÷è øEQUATION1168 V1 EN (Equation 57)where:Qn is the calculated present temperature,Qn-1 is the calculated temperature at the previous time step,Qfinal is the calculated final temperature with the actual current,Dt is the time step between calculation of the actual temperature andt is the set thermal time constant for the protected device (line or cable)The calculated component temperature is available as a real figure signal, TEMP.1MRK 506 335-UUS A Section 7Current protection253Technical manual
